[PATCH mm-unstable v1 0/7] mm: multi-gen LRU: improve

[PATCH mm-unstable v1 0/7] mm: multi-gen LRU: improve

[T.J. Alumbaugh]

This patch series improves a few MGLRU functions, collects related
functions, and adds additional documentation.

T.J. Alumbaugh (7):
  mm: multi-gen LRU: section for working set protection
  mm: multi-gen LRU: section for rmap/PT walk feedback
  mm: multi-gen LRU: section for Bloom filters
  mm: multi-gen LRU: section for memcg LRU
  mm: multi-gen LRU: improve lru_gen_exit_memcg()
  mm: multi-gen LRU: improve walk_pmd_range()
  mm: multi-gen LRU: simplify lru_gen_look_around()

 Documentation/mm/multigen_lru.rst |  78 ++++-
 include/linux/mm_inline.h         |  17 -
 include/linux/mmzone.h            |  13 +-
 mm/memcontrol.c                   |   8 +-
 mm/vmscan.c                       | 534 ++++++++++++++++--------------
 5 files changed, 360 insertions(+), 290 deletions(-)

-- 
2.39.0.314.g84b9a713c41-goog

[PATCH mm-unstable v1 1/7] mm: multi-gen LRU: section for working set protection

[T.J. Alumbaugh]

Add a section for working set protection in the code and the design
doc. The admin doc already contains its usage.

Signed-off-by: T.J. Alumbaugh <talumbau@google.com>
---
 Documentation/mm/multigen_lru.rst | 15 +++++++++++++++
 mm/vmscan.c                       |  4 ++++
 2 files changed, 19 insertions(+)

diff --git a/Documentation/mm/multigen_lru.rst b/Documentation/mm/multigen_lru.rst
index d8f721f98868..6e1483e70fdc 100644
--- a/Documentation/mm/multigen_lru.rst
+++ b/Documentation/mm/multigen_lru.rst
@@ -141,6 +141,21 @@ loop has detected outlying refaults from the tier this page is in. To
 this end, the feedback loop uses the first tier as the baseline, for
 the reason stated earlier.
 
+Working set protection
+----------------------
+Each generation is timestamped at birth. If ``lru_gen_min_ttl`` is
+set, an ``lruvec`` is protected from the eviction when its oldest
+generation was born within ``lru_gen_min_ttl`` milliseconds. In other
+words, it prevents the working set of ``lru_gen_min_ttl`` milliseconds
+from getting evicted. The OOM killer is triggered if this working set
+cannot be kept in memory.
+
+This time-based approach has the following advantages:
+
+1. It is easier to configure because it is agnostic to applications
+   and memory sizes.
+2. It is more reliable because it is directly wired to the OOM killer.
+
 Summary
 -------
 The multi-gen LRU can be disassembled into the following parts:
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 394ff4962cbc..a741765896b6 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4475,6 +4475,10 @@ static bool try_to_inc_max_seq(struct lruvec *lruvec, unsigned long max_seq,
 	return true;
 }
 
+/******************************************************************************
+ *                          working set protection
+ ******************************************************************************/
+
 static bool lruvec_is_sizable(struct lruvec *lruvec, struct scan_control *sc)
 {
 	int gen, type, zone;
-- 
2.39.0.314.g84b9a713c41-goog

Re: [PATCH mm-unstable v1 1/7] mm: multi-gen LRU: section for working set protection

[Matthew Wilcox]

On Wed, Jan 18, 2023 at 12:18:21AM +0000, T.J. Alumbaugh wrote:
> +++ b/mm/vmscan.c
> @@ -4475,6 +4475,10 @@ static bool try_to_inc_max_seq(struct lruvec *lruvec, unsigned long max_seq,
>  	return true;
>  }
>  
> +/******************************************************************************
> + *                          working set protection
> + ******************************************************************************/
> +
>  static bool lruvec_is_sizable(struct lruvec *lruvec, struct scan_control *sc)

We don't usually do this.  Why do you want to?

[PATCH mm-unstable v1 2/7] mm: multi-gen LRU: section for rmap/PT walk feedback

[T.J. Alumbaugh]

Add a section for lru_gen_look_around() in the code and the design
doc.

Signed-off-by: T.J. Alumbaugh <talumbau@google.com>
---
 Documentation/mm/multigen_lru.rst | 14 ++++++++++++++
 mm/vmscan.c                       |  4 ++++
 2 files changed, 18 insertions(+)

diff --git a/Documentation/mm/multigen_lru.rst b/Documentation/mm/multigen_lru.rst
index 6e1483e70fdc..bd988a142bc2 100644
--- a/Documentation/mm/multigen_lru.rst
+++ b/Documentation/mm/multigen_lru.rst
@@ -156,6 +156,20 @@ This time-based approach has the following advantages:
    and memory sizes.
 2. It is more reliable because it is directly wired to the OOM killer.
 
+Rmap/PT walk feedback
+---------------------
+Searching the rmap for PTEs mapping each page on an LRU list (to test
+and clear the accessed bit) can be expensive because pages from
+different VMAs (PA space) are not cache friendly to the rmap (VA
+space). For workloads mostly using mapped pages, searching the rmap
+can incur the highest CPU cost in the reclaim path.
+
+``lru_gen_look_around()`` exploits spatial locality to reduce the
+trips into the rmap. It scans the adjacent PTEs of a young PTE and
+promotes hot pages. If the scan was done cacheline efficiently, it
+adds the PMD entry pointing to the PTE table to the Bloom filter. This
+forms a feedback loop between the eviction and the aging.
+
 Summary
 -------
 The multi-gen LRU can be disassembled into the following parts:
diff --git a/mm/vmscan.c b/mm/vmscan.c
index a741765896b6..eb9263bf6806 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4569,6 +4569,10 @@ static void lru_gen_age_node(struct pglist_data *pgdat, struct scan_control *sc)
 	}
 }
 
+/******************************************************************************
+ *                          rmap/PT walk feedback
+ ******************************************************************************/
+
 /*
  * This function exploits spatial locality when shrink_folio_list() walks the
  * rmap. It scans the adjacent PTEs of a young PTE and promotes hot pages. If
-- 
2.39.0.314.g84b9a713c41-goog

[PATCH mm-unstable v1 3/7] mm: multi-gen LRU: section for Bloom filters

[T.J. Alumbaugh]

Move Bloom filters code into a dedicated section. Improve the design
doc to explain Bloom filter usage and connection between aging and
eviction in their use.

Signed-off-by: T.J. Alumbaugh <talumbau@google.com>
---
 Documentation/mm/multigen_lru.rst |  16 +++
 mm/vmscan.c                       | 180 +++++++++++++++---------------
 2 files changed, 108 insertions(+), 88 deletions(-)

diff --git a/Documentation/mm/multigen_lru.rst b/Documentation/mm/multigen_lru.rst
index bd988a142bc2..770b5d539856 100644
--- a/Documentation/mm/multigen_lru.rst
+++ b/Documentation/mm/multigen_lru.rst
@@ -170,6 +170,22 @@ promotes hot pages. If the scan was done cacheline efficiently, it
 adds the PMD entry pointing to the PTE table to the Bloom filter. This
 forms a feedback loop between the eviction and the aging.
 
+Bloom Filters
+-------------
+Bloom filters are a space and memory efficient data structure for set
+membership test, i.e., test if an element is not in the set or may be
+in the set.
+
+In the eviction path, specifically, in ``lru_gen_look_around()``, if a
+PMD has a sufficient number of hot pages, its address is placed in the
+filter. In the aging path, set membership means that the PTE range
+will be scanned for young pages.
+
+Note that Bloom filters are probabilistic on set membership. If a test
+is false positive, the cost is an additional scan of a range of PTEs,
+which may yield hot pages anyway. Parameters of the filter itself can
+control the false positive rate in the limit.
+
 Summary
 -------
 The multi-gen LRU can be disassembled into the following parts:
diff --git a/mm/vmscan.c b/mm/vmscan.c
index eb9263bf6806..1be9120349f8 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3233,6 +3233,98 @@ static bool __maybe_unused seq_is_valid(struct lruvec *lruvec)
 	       get_nr_gens(lruvec, LRU_GEN_ANON) <= MAX_NR_GENS;
 }
 
+/******************************************************************************
+ *                          Bloom filters
+ ******************************************************************************/
+
+/*
+ * Bloom filters with m=1<<15, k=2 and the false positive rates of ~1/5 when
+ * n=10,000 and ~1/2 when n=20,000, where, conventionally, m is the number of
+ * bits in a bitmap, k is the number of hash functions and n is the number of
+ * inserted items.
+ *
+ * Page table walkers use one of the two filters to reduce their search space.
+ * To get rid of non-leaf entries that no longer have enough leaf entries, the
+ * aging uses the double-buffering technique to flip to the other filter each
+ * time it produces a new generation. For non-leaf entries that have enough
+ * leaf entries, the aging carries them over to the next generation in
+ * walk_pmd_range(); the eviction also report them when walking the rmap
+ * in lru_gen_look_around().
+ *
+ * For future optimizations:
+ * 1. It's not necessary to keep both filters all the time. The spare one can be
+ *    freed after the RCU grace period and reallocated if needed again.
+ * 2. And when reallocating, it's worth scaling its size according to the number
+ *    of inserted entries in the other filter, to reduce the memory overhead on
+ *    small systems and false positives on large systems.
+ * 3. Jenkins' hash function is an alternative to Knuth's.
+ */
+#define BLOOM_FILTER_SHIFT	15
+
+static inline int filter_gen_from_seq(unsigned long seq)
+{
+	return seq % NR_BLOOM_FILTERS;
+}
+
+static void get_item_key(void *item, int *key)
+{
+	u32 hash = hash_ptr(item, BLOOM_FILTER_SHIFT * 2);
+
+	BUILD_BUG_ON(BLOOM_FILTER_SHIFT * 2 > BITS_PER_TYPE(u32));
+
+	key[0] = hash & (BIT(BLOOM_FILTER_SHIFT) - 1);
+	key[1] = hash >> BLOOM_FILTER_SHIFT;
+}
+
+static bool test_bloom_filter(struct lruvec *lruvec, unsigned long seq, void *item)
+{
+	int key[2];
+	unsigned long *filter;
+	int gen = filter_gen_from_seq(seq);
+
+	filter = READ_ONCE(lruvec->mm_state.filters[gen]);
+	if (!filter)
+		return true;
+
+	get_item_key(item, key);
+
+	return test_bit(key[0], filter) && test_bit(key[1], filter);
+}
+
+static void update_bloom_filter(struct lruvec *lruvec, unsigned long seq, void *item)
+{
+	int key[2];
+	unsigned long *filter;
+	int gen = filter_gen_from_seq(seq);
+
+	filter = READ_ONCE(lruvec->mm_state.filters[gen]);
+	if (!filter)
+		return;
+
+	get_item_key(item, key);
+
+	if (!test_bit(key[0], filter))
+		set_bit(key[0], filter);
+	if (!test_bit(key[1], filter))
+		set_bit(key[1], filter);
+}
+
+static void reset_bloom_filter(struct lruvec *lruvec, unsigned long seq)
+{
+	unsigned long *filter;
+	int gen = filter_gen_from_seq(seq);
+
+	filter = lruvec->mm_state.filters[gen];
+	if (filter) {
+		bitmap_clear(filter, 0, BIT(BLOOM_FILTER_SHIFT));
+		return;
+	}
+
+	filter = bitmap_zalloc(BIT(BLOOM_FILTER_SHIFT),
+			       __GFP_HIGH | __GFP_NOMEMALLOC | __GFP_NOWARN);
+	WRITE_ONCE(lruvec->mm_state.filters[gen], filter);
+}
+
 /******************************************************************************
  *                          mm_struct list
  ******************************************************************************/
@@ -3352,94 +3444,6 @@ void lru_gen_migrate_mm(struct mm_struct *mm)
 }
 #endif
 
-/*
- * Bloom filters with m=1<<15, k=2 and the false positive rates of ~1/5 when
- * n=10,000 and ~1/2 when n=20,000, where, conventionally, m is the number of
- * bits in a bitmap, k is the number of hash functions and n is the number of
- * inserted items.
- *
- * Page table walkers use one of the two filters to reduce their search space.
- * To get rid of non-leaf entries that no longer have enough leaf entries, the
- * aging uses the double-buffering technique to flip to the other filter each
- * time it produces a new generation. For non-leaf entries that have enough
- * leaf entries, the aging carries them over to the next generation in
- * walk_pmd_range(); the eviction also report them when walking the rmap
- * in lru_gen_look_around().
- *
- * For future optimizations:
- * 1. It's not necessary to keep both filters all the time. The spare one can be
- *    freed after the RCU grace period and reallocated if needed again.
- * 2. And when reallocating, it's worth scaling its size according to the number
- *    of inserted entries in the other filter, to reduce the memory overhead on
- *    small systems and false positives on large systems.
- * 3. Jenkins' hash function is an alternative to Knuth's.
- */
-#define BLOOM_FILTER_SHIFT	15
-
-static inline int filter_gen_from_seq(unsigned long seq)
-{
-	return seq % NR_BLOOM_FILTERS;
-}
-
-static void get_item_key(void *item, int *key)
-{
-	u32 hash = hash_ptr(item, BLOOM_FILTER_SHIFT * 2);
-
-	BUILD_BUG_ON(BLOOM_FILTER_SHIFT * 2 > BITS_PER_TYPE(u32));
-
-	key[0] = hash & (BIT(BLOOM_FILTER_SHIFT) - 1);
-	key[1] = hash >> BLOOM_FILTER_SHIFT;
-}
-
-static void reset_bloom_filter(struct lruvec *lruvec, unsigned long seq)
-{
-	unsigned long *filter;
-	int gen = filter_gen_from_seq(seq);
-
-	filter = lruvec->mm_state.filters[gen];
-	if (filter) {
-		bitmap_clear(filter, 0, BIT(BLOOM_FILTER_SHIFT));
-		return;
-	}
-
-	filter = bitmap_zalloc(BIT(BLOOM_FILTER_SHIFT),
-			       __GFP_HIGH | __GFP_NOMEMALLOC | __GFP_NOWARN);
-	WRITE_ONCE(lruvec->mm_state.filters[gen], filter);
-}
-
-static void update_bloom_filter(struct lruvec *lruvec, unsigned long seq, void *item)
-{
-	int key[2];
-	unsigned long *filter;
-	int gen = filter_gen_from_seq(seq);
-
-	filter = READ_ONCE(lruvec->mm_state.filters[gen]);
-	if (!filter)
-		return;
-
-	get_item_key(item, key);
-
-	if (!test_bit(key[0], filter))
-		set_bit(key[0], filter);
-	if (!test_bit(key[1], filter))
-		set_bit(key[1], filter);
-}
-
-static bool test_bloom_filter(struct lruvec *lruvec, unsigned long seq, void *item)
-{
-	int key[2];
-	unsigned long *filter;
-	int gen = filter_gen_from_seq(seq);
-
-	filter = READ_ONCE(lruvec->mm_state.filters[gen]);
-	if (!filter)
-		return true;
-
-	get_item_key(item, key);
-
-	return test_bit(key[0], filter) && test_bit(key[1], filter);
-}
-
 static void reset_mm_stats(struct lruvec *lruvec, struct lru_gen_mm_walk *walk, bool last)
 {
 	int i;
-- 
2.39.0.314.g84b9a713c41-goog

[PATCH mm-unstable v1 4/7] mm: multi-gen LRU: section for memcg LRU

[T.J. Alumbaugh]

Move memcg LRU code into a dedicated section. Improve the design
doc to outline its architecture.

Signed-off-by: T.J. Alumbaugh <talumbau@google.com>
---
 Documentation/mm/multigen_lru.rst |  33 +++-
 include/linux/mm_inline.h         |  17 --
 include/linux/mmzone.h            |  13 +-
 mm/memcontrol.c                   |   8 +-
 mm/vmscan.c                       | 250 +++++++++++++++++-------------
 5 files changed, 178 insertions(+), 143 deletions(-)

diff --git a/Documentation/mm/multigen_lru.rst b/Documentation/mm/multigen_lru.rst
index 770b5d539856..5f1f6ecbb79b 100644
--- a/Documentation/mm/multigen_lru.rst
+++ b/Documentation/mm/multigen_lru.rst
@@ -186,9 +186,40 @@ is false positive, the cost is an additional scan of a range of PTEs,
 which may yield hot pages anyway. Parameters of the filter itself can
 control the false positive rate in the limit.
 
+Memcg LRU
+---------
+An memcg LRU is a per-node LRU of memcgs. It is also an LRU of LRUs,
+since each node and memcg combination has an LRU of folios (see
+``mem_cgroup_lruvec()``). Its goal is to improve the scalability of
+global reclaim, which is critical to system-wide memory overcommit in
+data centers. Note that memcg LRU only applies to global reclaim.
+
+The basic structure of an memcg LRU can be understood by an analogy to
+the active/inactive LRU (of folios):
+
+1. It has the young and the old (generations), i.e., the counterparts
+   to the active and the inactive;
+2. The increment of ``max_seq`` triggers promotion, i.e., the
+   counterpart to activation;
+3. Other events trigger similar operations, e.g., offlining an memcg
+   triggers demotion, i.e., the counterpart to deactivation.
+
+In terms of global reclaim, it has two distinct features:
+
+1. Sharding, which allows each thread to start at a random memcg (in
+   the old generation) and improves parallelism;
+2. Eventual fairness, which allows direct reclaim to bail out at will
+   and reduces latency without affecting fairness over some time.
+
+In terms of traversing memcgs during global reclaim, it improves the
+best-case complexity from O(n) to O(1) and does not affect the
+worst-case complexity O(n). Therefore, on average, it has a sublinear
+complexity.
+
 Summary
 -------
-The multi-gen LRU can be disassembled into the following parts:
+The multi-gen LRU (of folios) can be disassembled into the following
+parts:
 
 * Generations
 * Rmap walks
diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
index 26dcbda07e92..de1e622dd366 100644
--- a/include/linux/mm_inline.h
+++ b/include/linux/mm_inline.h
@@ -122,18 +122,6 @@ static inline bool lru_gen_in_fault(void)
 	return current->in_lru_fault;
 }
 
-#ifdef CONFIG_MEMCG
-static inline int lru_gen_memcg_seg(struct lruvec *lruvec)
-{
-	return READ_ONCE(lruvec->lrugen.seg);
-}
-#else
-static inline int lru_gen_memcg_seg(struct lruvec *lruvec)
-{
-	return 0;
-}
-#endif
-
 static inline int lru_gen_from_seq(unsigned long seq)
 {
 	return seq % MAX_NR_GENS;
@@ -309,11 +297,6 @@ static inline bool lru_gen_in_fault(void)
 	return false;
 }
 
-static inline int lru_gen_memcg_seg(struct lruvec *lruvec)
-{
-	return 0;
-}
-
 static inline bool lru_gen_add_folio(struct lruvec *lruvec, struct folio *folio, bool reclaiming)
 {
 	return false;
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index ae7d4e92c12d..c54964979ccf 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -368,15 +368,6 @@ struct page_vma_mapped_walk;
 #define LRU_GEN_MASK		((BIT(LRU_GEN_WIDTH) - 1) << LRU_GEN_PGOFF)
 #define LRU_REFS_MASK		((BIT(LRU_REFS_WIDTH) - 1) << LRU_REFS_PGOFF)
 
-/* see the comment on MEMCG_NR_GENS */
-enum {
-	MEMCG_LRU_NOP,
-	MEMCG_LRU_HEAD,
-	MEMCG_LRU_TAIL,
-	MEMCG_LRU_OLD,
-	MEMCG_LRU_YOUNG,
-};
-
 #ifdef CONFIG_LRU_GEN
 
 enum {
@@ -557,7 +548,7 @@ void lru_gen_exit_memcg(struct mem_cgroup *memcg);
 void lru_gen_online_memcg(struct mem_cgroup *memcg);
 void lru_gen_offline_memcg(struct mem_cgroup *memcg);
 void lru_gen_release_memcg(struct mem_cgroup *memcg);
-void lru_gen_rotate_memcg(struct lruvec *lruvec, int op);
+void lru_gen_soft_reclaim(struct lruvec *lruvec);
 
 #else /* !CONFIG_MEMCG */
 
@@ -608,7 +599,7 @@ static inline void lru_gen_release_memcg(struct mem_cgroup *memcg)
 {
 }
 
-static inline void lru_gen_rotate_memcg(struct lruvec *lruvec, int op)
+static inline void lru_gen_soft_reclaim(struct lruvec *lruvec)
 {
 }
 
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 893427aded01..17335459d8dc 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -476,12 +476,8 @@ static void mem_cgroup_update_tree(struct mem_cgroup *memcg, int nid)
 	struct mem_cgroup_tree_per_node *mctz;
 
 	if (lru_gen_enabled()) {
-		struct lruvec *lruvec = &memcg->nodeinfo[nid]->lruvec;
-
-		/* see the comment on MEMCG_NR_GENS */
-		if (soft_limit_excess(memcg) && lru_gen_memcg_seg(lruvec) != MEMCG_LRU_HEAD)
-			lru_gen_rotate_memcg(lruvec, MEMCG_LRU_HEAD);
-
+		if (soft_limit_excess(memcg))
+			lru_gen_soft_reclaim(&memcg->nodeinfo[nid]->lruvec);
 		return;
 	}
 
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 1be9120349f8..796d4ca65e97 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4705,6 +4705,148 @@ void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
 	mem_cgroup_unlock_pages();
 }
 
+/******************************************************************************
+ *                          memcg LRU
+ ******************************************************************************/
+
+/* see the comment on MEMCG_NR_GENS */
+enum {
+	MEMCG_LRU_NOP,
+	MEMCG_LRU_HEAD,
+	MEMCG_LRU_TAIL,
+	MEMCG_LRU_OLD,
+	MEMCG_LRU_YOUNG,
+};
+
+#ifdef CONFIG_MEMCG
+
+static int lru_gen_memcg_seg(struct lruvec *lruvec)
+{
+	return READ_ONCE(lruvec->lrugen.seg);
+}
+
+static void lru_gen_rotate_memcg(struct lruvec *lruvec, int op)
+{
+	int seg;
+	int old, new;
+	int bin = get_random_u32_below(MEMCG_NR_BINS);
+	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
+
+	spin_lock(&pgdat->memcg_lru.lock);
+
+	VM_WARN_ON_ONCE(hlist_nulls_unhashed(&lruvec->lrugen.list));
+
+	seg = 0;
+	new = old = lruvec->lrugen.gen;
+
+	/* see the comment on MEMCG_NR_GENS */
+	if (op == MEMCG_LRU_HEAD)
+		seg = MEMCG_LRU_HEAD;
+	else if (op == MEMCG_LRU_TAIL)
+		seg = MEMCG_LRU_TAIL;
+	else if (op == MEMCG_LRU_OLD)
+		new = get_memcg_gen(pgdat->memcg_lru.seq);
+	else if (op == MEMCG_LRU_YOUNG)
+		new = get_memcg_gen(pgdat->memcg_lru.seq + 1);
+	else
+		VM_WARN_ON_ONCE(true);
+
+	hlist_nulls_del_rcu(&lruvec->lrugen.list);
+
+	if (op == MEMCG_LRU_HEAD || op == MEMCG_LRU_OLD)
+		hlist_nulls_add_head_rcu(&lruvec->lrugen.list, &pgdat->memcg_lru.fifo[new][bin]);
+	else
+		hlist_nulls_add_tail_rcu(&lruvec->lrugen.list, &pgdat->memcg_lru.fifo[new][bin]);
+
+	pgdat->memcg_lru.nr_memcgs[old]--;
+	pgdat->memcg_lru.nr_memcgs[new]++;
+
+	lruvec->lrugen.gen = new;
+	WRITE_ONCE(lruvec->lrugen.seg, seg);
+
+	if (!pgdat->memcg_lru.nr_memcgs[old] && old == get_memcg_gen(pgdat->memcg_lru.seq))
+		WRITE_ONCE(pgdat->memcg_lru.seq, pgdat->memcg_lru.seq + 1);
+
+	spin_unlock(&pgdat->memcg_lru.lock);
+}
+
+void lru_gen_online_memcg(struct mem_cgroup *memcg)
+{
+	int gen;
+	int nid;
+	int bin = get_random_u32_below(MEMCG_NR_BINS);
+
+	for_each_node(nid) {
+		struct pglist_data *pgdat = NODE_DATA(nid);
+		struct lruvec *lruvec = get_lruvec(memcg, nid);
+
+		spin_lock(&pgdat->memcg_lru.lock);
+
+		VM_WARN_ON_ONCE(!hlist_nulls_unhashed(&lruvec->lrugen.list));
+
+		gen = get_memcg_gen(pgdat->memcg_lru.seq);
+
+		hlist_nulls_add_tail_rcu(&lruvec->lrugen.list, &pgdat->memcg_lru.fifo[gen][bin]);
+		pgdat->memcg_lru.nr_memcgs[gen]++;
+
+		lruvec->lrugen.gen = gen;
+
+		spin_unlock(&pgdat->memcg_lru.lock);
+	}
+}
+
+void lru_gen_offline_memcg(struct mem_cgroup *memcg)
+{
+	int nid;
+
+	for_each_node(nid) {
+		struct lruvec *lruvec = get_lruvec(memcg, nid);
+
+		lru_gen_rotate_memcg(lruvec, MEMCG_LRU_OLD);
+	}
+}
+
+void lru_gen_release_memcg(struct mem_cgroup *memcg)
+{
+	int gen;
+	int nid;
+
+	for_each_node(nid) {
+		struct pglist_data *pgdat = NODE_DATA(nid);
+		struct lruvec *lruvec = get_lruvec(memcg, nid);
+
+		spin_lock(&pgdat->memcg_lru.lock);
+
+		VM_WARN_ON_ONCE(hlist_nulls_unhashed(&lruvec->lrugen.list));
+
+		gen = lruvec->lrugen.gen;
+
+		hlist_nulls_del_rcu(&lruvec->lrugen.list);
+		pgdat->memcg_lru.nr_memcgs[gen]--;
+
+		if (!pgdat->memcg_lru.nr_memcgs[gen] && gen == get_memcg_gen(pgdat->memcg_lru.seq))
+			WRITE_ONCE(pgdat->memcg_lru.seq, pgdat->memcg_lru.seq + 1);
+
+		spin_unlock(&pgdat->memcg_lru.lock);
+	}
+}
+
+void lru_gen_soft_reclaim(struct lruvec *lruvec)
+{
+	/* see the comment on MEMCG_NR_GENS */
+	if (lru_gen_memcg_seg(lruvec) != MEMCG_LRU_HEAD)
+		lru_gen_rotate_memcg(lruvec, MEMCG_LRU_HEAD);
+}
+
+#else /* !CONFIG_MEMCG */
+
+static int lru_gen_memcg_seg(struct lruvec *lruvec)
+{
+	return 0;
+}
+
+#endif
+
 /******************************************************************************
  *                          the eviction
  ******************************************************************************/
@@ -5397,53 +5539,6 @@ static void lru_gen_shrink_node(struct pglist_data *pgdat, struct scan_control *
 	pgdat->kswapd_failures = 0;
 }
 
-#ifdef CONFIG_MEMCG
-void lru_gen_rotate_memcg(struct lruvec *lruvec, int op)
-{
-	int seg;
-	int old, new;
-	int bin = get_random_u32_below(MEMCG_NR_BINS);
-	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
-
-	spin_lock(&pgdat->memcg_lru.lock);
-
-	VM_WARN_ON_ONCE(hlist_nulls_unhashed(&lruvec->lrugen.list));
-
-	seg = 0;
-	new = old = lruvec->lrugen.gen;
-
-	/* see the comment on MEMCG_NR_GENS */
-	if (op == MEMCG_LRU_HEAD)
-		seg = MEMCG_LRU_HEAD;
-	else if (op == MEMCG_LRU_TAIL)
-		seg = MEMCG_LRU_TAIL;
-	else if (op == MEMCG_LRU_OLD)
-		new = get_memcg_gen(pgdat->memcg_lru.seq);
-	else if (op == MEMCG_LRU_YOUNG)
-		new = get_memcg_gen(pgdat->memcg_lru.seq + 1);
-	else
-		VM_WARN_ON_ONCE(true);
-
-	hlist_nulls_del_rcu(&lruvec->lrugen.list);
-
-	if (op == MEMCG_LRU_HEAD || op == MEMCG_LRU_OLD)
-		hlist_nulls_add_head_rcu(&lruvec->lrugen.list, &pgdat->memcg_lru.fifo[new][bin]);
-	else
-		hlist_nulls_add_tail_rcu(&lruvec->lrugen.list, &pgdat->memcg_lru.fifo[new][bin]);
-
-	pgdat->memcg_lru.nr_memcgs[old]--;
-	pgdat->memcg_lru.nr_memcgs[new]++;
-
-	lruvec->lrugen.gen = new;
-	WRITE_ONCE(lruvec->lrugen.seg, seg);
-
-	if (!pgdat->memcg_lru.nr_memcgs[old] && old == get_memcg_gen(pgdat->memcg_lru.seq))
-		WRITE_ONCE(pgdat->memcg_lru.seq, pgdat->memcg_lru.seq + 1);
-
-	spin_unlock(&pgdat->memcg_lru.lock);
-}
-#endif
-
 /******************************************************************************
  *                          state change
  ******************************************************************************/
@@ -6086,67 +6181,6 @@ void lru_gen_exit_memcg(struct mem_cgroup *memcg)
 	}
 }
 
-void lru_gen_online_memcg(struct mem_cgroup *memcg)
-{
-	int gen;
-	int nid;
-	int bin = get_random_u32_below(MEMCG_NR_BINS);
-
-	for_each_node(nid) {
-		struct pglist_data *pgdat = NODE_DATA(nid);
-		struct lruvec *lruvec = get_lruvec(memcg, nid);
-
-		spin_lock(&pgdat->memcg_lru.lock);
-
-		VM_WARN_ON_ONCE(!hlist_nulls_unhashed(&lruvec->lrugen.list));
-
-		gen = get_memcg_gen(pgdat->memcg_lru.seq);
-
-		hlist_nulls_add_tail_rcu(&lruvec->lrugen.list, &pgdat->memcg_lru.fifo[gen][bin]);
-		pgdat->memcg_lru.nr_memcgs[gen]++;
-
-		lruvec->lrugen.gen = gen;
-
-		spin_unlock(&pgdat->memcg_lru.lock);
-	}
-}
-
-void lru_gen_offline_memcg(struct mem_cgroup *memcg)
-{
-	int nid;
-
-	for_each_node(nid) {
-		struct lruvec *lruvec = get_lruvec(memcg, nid);
-
-		lru_gen_rotate_memcg(lruvec, MEMCG_LRU_OLD);
-	}
-}
-
-void lru_gen_release_memcg(struct mem_cgroup *memcg)
-{
-	int gen;
-	int nid;
-
-	for_each_node(nid) {
-		struct pglist_data *pgdat = NODE_DATA(nid);
-		struct lruvec *lruvec = get_lruvec(memcg, nid);
-
-		spin_lock(&pgdat->memcg_lru.lock);
-
-		VM_WARN_ON_ONCE(hlist_nulls_unhashed(&lruvec->lrugen.list));
-
-		gen = lruvec->lrugen.gen;
-
-		hlist_nulls_del_rcu(&lruvec->lrugen.list);
-		pgdat->memcg_lru.nr_memcgs[gen]--;
-
-		if (!pgdat->memcg_lru.nr_memcgs[gen] && gen == get_memcg_gen(pgdat->memcg_lru.seq))
-			WRITE_ONCE(pgdat->memcg_lru.seq, pgdat->memcg_lru.seq + 1);
-
-		spin_unlock(&pgdat->memcg_lru.lock);
-	}
-}
-
 #endif /* CONFIG_MEMCG */
 
 static int __init init_lru_gen(void)
-- 
2.39.0.314.g84b9a713c41-goog

[PATCH mm-unstable v1 5/7] mm: multi-gen LRU: improve lru_gen_exit_memcg()

[T.J. Alumbaugh]

Add warnings and poison ->next.

Signed-off-by: T.J. Alumbaugh <talumbau@google.com>
---
 mm/vmscan.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 796d4ca65e97..c2e6ad53447b 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -6168,12 +6168,17 @@ void lru_gen_exit_memcg(struct mem_cgroup *memcg)
 	int i;
 	int nid;
 
+	VM_WARN_ON_ONCE(!list_empty(&memcg->mm_list.fifo));
+
 	for_each_node(nid) {
 		struct lruvec *lruvec = get_lruvec(memcg, nid);
 
+		VM_WARN_ON_ONCE(lruvec->mm_state.nr_walkers);
 		VM_WARN_ON_ONCE(memchr_inv(lruvec->lrugen.nr_pages, 0,
 					   sizeof(lruvec->lrugen.nr_pages)));
 
+		lruvec->lrugen.list.next = LIST_POISON1;
+
 		for (i = 0; i < NR_BLOOM_FILTERS; i++) {
 			bitmap_free(lruvec->mm_state.filters[i]);
 			lruvec->mm_state.filters[i] = NULL;
-- 
2.39.0.314.g84b9a713c41-goog

[PATCH mm-unstable v1 6/7] mm: multi-gen LRU: improve walk_pmd_range()

[T.J. Alumbaugh]

Improve readability of walk_pmd_range() and walk_pmd_range_locked().

Signed-off-by: T.J. Alumbaugh <talumbau@google.com>
---
 mm/vmscan.c | 40 ++++++++++++++++++++--------------------
 1 file changed, 20 insertions(+), 20 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index c2e6ad53447b..ff3b4aa3c31f 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3999,8 +3999,8 @@ static bool walk_pte_range(pmd_t *pmd, unsigned long start, unsigned long end,
 }
 
 #if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG)
-static void walk_pmd_range_locked(pud_t *pud, unsigned long next, struct vm_area_struct *vma,
-				  struct mm_walk *args, unsigned long *bitmap, unsigned long *start)
+static void walk_pmd_range_locked(pud_t *pud, unsigned long addr, struct vm_area_struct *vma,
+				  struct mm_walk *args, unsigned long *bitmap, unsigned long *first)
 {
 	int i;
 	pmd_t *pmd;
@@ -4013,18 +4013,19 @@ static void walk_pmd_range_locked(pud_t *pud, unsigned long next, struct vm_area
 	VM_WARN_ON_ONCE(pud_leaf(*pud));
 
 	/* try to batch at most 1+MIN_LRU_BATCH+1 entries */
-	if (*start == -1) {
-		*start = next;
+	if (*first == -1) {
+		*first = addr;
+		bitmap_zero(bitmap, MIN_LRU_BATCH);
 		return;
 	}
 
-	i = next == -1 ? 0 : pmd_index(next) - pmd_index(*start);
+	i = addr == -1 ? 0 : pmd_index(addr) - pmd_index(*first);
 	if (i && i <= MIN_LRU_BATCH) {
 		__set_bit(i - 1, bitmap);
 		return;
 	}
 
-	pmd = pmd_offset(pud, *start);
+	pmd = pmd_offset(pud, *first);
 
 	ptl = pmd_lockptr(args->mm, pmd);
 	if (!spin_trylock(ptl))
@@ -4035,15 +4036,16 @@ static void walk_pmd_range_locked(pud_t *pud, unsigned long next, struct vm_area
 	do {
 		unsigned long pfn;
 		struct folio *folio;
-		unsigned long addr = i ? (*start & PMD_MASK) + i * PMD_SIZE : *start;
+
+		/* don't round down the first address */
+		addr = i ? (*first & PMD_MASK) + i * PMD_SIZE : *first;
 
 		pfn = get_pmd_pfn(pmd[i], vma, addr);
 		if (pfn == -1)
 			goto next;
 
 		if (!pmd_trans_huge(pmd[i])) {
-			if (arch_has_hw_nonleaf_pmd_young() &&
-			    get_cap(LRU_GEN_NONLEAF_YOUNG))
+			if (arch_has_hw_nonleaf_pmd_young() && get_cap(LRU_GEN_NONLEAF_YOUNG))
 				pmdp_test_and_clear_young(vma, addr, pmd + i);
 			goto next;
 		}
@@ -4072,12 +4074,11 @@ static void walk_pmd_range_locked(pud_t *pud, unsigned long next, struct vm_area
 	arch_leave_lazy_mmu_mode();
 	spin_unlock(ptl);
 done:
-	*start = -1;
-	bitmap_zero(bitmap, MIN_LRU_BATCH);
+	*first = -1;
 }
 #else
-static void walk_pmd_range_locked(pud_t *pud, unsigned long next, struct vm_area_struct *vma,
-				  struct mm_walk *args, unsigned long *bitmap, unsigned long *start)
+static void walk_pmd_range_locked(pud_t *pud, unsigned long addr, struct vm_area_struct *vma,
+				  struct mm_walk *args, unsigned long *bitmap, unsigned long *first)
 {
 }
 #endif
@@ -4090,9 +4091,9 @@ static void walk_pmd_range(pud_t *pud, unsigned long start, unsigned long end,
 	unsigned long next;
 	unsigned long addr;
 	struct vm_area_struct *vma;
-	unsigned long pos = -1;
+	unsigned long bitmap[BITS_TO_LONGS(MIN_LRU_BATCH)];
+	unsigned long first = -1;
 	struct lru_gen_mm_walk *walk = args->private;
-	unsigned long bitmap[BITS_TO_LONGS(MIN_LRU_BATCH)] = {};
 
 	VM_WARN_ON_ONCE(pud_leaf(*pud));
 
@@ -4131,18 +4132,17 @@ static void walk_pmd_range(pud_t *pud, unsigned long start, unsigned long end,
 			if (pfn < pgdat->node_start_pfn || pfn >= pgdat_end_pfn(pgdat))
 				continue;
 
-			walk_pmd_range_locked(pud, addr, vma, args, bitmap, &pos);
+			walk_pmd_range_locked(pud, addr, vma, args, bitmap, &first);
 			continue;
 		}
 #endif
 		walk->mm_stats[MM_NONLEAF_TOTAL]++;
 
-		if (arch_has_hw_nonleaf_pmd_young() &&
-		    get_cap(LRU_GEN_NONLEAF_YOUNG)) {
+		if (arch_has_hw_nonleaf_pmd_young() && get_cap(LRU_GEN_NONLEAF_YOUNG)) {
 			if (!pmd_young(val))
 				continue;
 
-			walk_pmd_range_locked(pud, addr, vma, args, bitmap, &pos);
+			walk_pmd_range_locked(pud, addr, vma, args, bitmap, &first);
 		}
 
 		if (!walk->force_scan && !test_bloom_filter(walk->lruvec, walk->max_seq, pmd + i))
@@ -4159,7 +4159,7 @@ static void walk_pmd_range(pud_t *pud, unsigned long start, unsigned long end,
 		update_bloom_filter(walk->lruvec, walk->max_seq + 1, pmd + i);
 	}
 
-	walk_pmd_range_locked(pud, -1, vma, args, bitmap, &pos);
+	walk_pmd_range_locked(pud, -1, vma, args, bitmap, &first);
 
 	if (i < PTRS_PER_PMD && get_next_vma(PUD_MASK, PMD_SIZE, args, &start, &end))
 		goto restart;
-- 
2.39.0.314.g84b9a713c41-goog

[PATCH mm-unstable v1 7/7] mm: multi-gen LRU: simplify lru_gen_look_around()

[T.J. Alumbaugh]

Update the folio generation in place with or without
current->reclaim_state->mm_walk. The LRU lock is held for longer, if
mm_walk is NULL and the number of folios to update is more than
PAGEVEC_SIZE.

This causes a measurable regression from the LRU lock contention
during a microbencmark. But a tiny regression is not worth the
complexity.

Signed-off-by: T.J. Alumbaugh <talumbau@google.com>
---
 mm/vmscan.c | 73 +++++++++++++++++------------------------------------
 1 file changed, 23 insertions(+), 50 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index ff3b4aa3c31f..ac51150d2d36 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4587,13 +4587,12 @@ static void lru_gen_age_node(struct pglist_data *pgdat, struct scan_control *sc)
 void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
 {
 	int i;
-	pte_t *pte;
 	unsigned long start;
 	unsigned long end;
-	unsigned long addr;
 	struct lru_gen_mm_walk *walk;
 	int young = 0;
-	unsigned long bitmap[BITS_TO_LONGS(MIN_LRU_BATCH)] = {};
+	pte_t *pte = pvmw->pte;
+	unsigned long addr = pvmw->address;
 	struct folio *folio = pfn_folio(pvmw->pfn);
 	struct mem_cgroup *memcg = folio_memcg(folio);
 	struct pglist_data *pgdat = folio_pgdat(folio);
@@ -4610,25 +4609,28 @@ void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
 	/* avoid taking the LRU lock under the PTL when possible */
 	walk = current->reclaim_state ? current->reclaim_state->mm_walk : NULL;
 
-	start = max(pvmw->address & PMD_MASK, pvmw->vma->vm_start);
-	end = min(pvmw->address | ~PMD_MASK, pvmw->vma->vm_end - 1) + 1;
+	start = max(addr & PMD_MASK, pvmw->vma->vm_start);
+	end = min(addr | ~PMD_MASK, pvmw->vma->vm_end - 1) + 1;
 
 	if (end - start > MIN_LRU_BATCH * PAGE_SIZE) {
-		if (pvmw->address - start < MIN_LRU_BATCH * PAGE_SIZE / 2)
+		if (addr - start < MIN_LRU_BATCH * PAGE_SIZE / 2)
 			end = start + MIN_LRU_BATCH * PAGE_SIZE;
-		else if (end - pvmw->address < MIN_LRU_BATCH * PAGE_SIZE / 2)
+		else if (end - addr < MIN_LRU_BATCH * PAGE_SIZE / 2)
 			start = end - MIN_LRU_BATCH * PAGE_SIZE;
 		else {
-			start = pvmw->address - MIN_LRU_BATCH * PAGE_SIZE / 2;
-			end = pvmw->address + MIN_LRU_BATCH * PAGE_SIZE / 2;
+			start = addr - MIN_LRU_BATCH * PAGE_SIZE / 2;
+			end = addr + MIN_LRU_BATCH * PAGE_SIZE / 2;
 		}
 	}
 
-	pte = pvmw->pte - (pvmw->address - start) / PAGE_SIZE;
+	/* folio_update_gen() requires stable folio_memcg() */
+	if (!mem_cgroup_trylock_pages(memcg))
+		return;
 
-	rcu_read_lock();
 	arch_enter_lazy_mmu_mode();
 
+	pte -= (addr - start) / PAGE_SIZE;
+
 	for (i = 0, addr = start; addr != end; i++, addr += PAGE_SIZE) {
 		unsigned long pfn;
 
@@ -4653,56 +4655,27 @@ void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
 		      !folio_test_swapcache(folio)))
 			folio_mark_dirty(folio);
 
+		if (walk) {
+			old_gen = folio_update_gen(folio, new_gen);
+			if (old_gen >= 0 && old_gen != new_gen)
+				update_batch_size(walk, folio, old_gen, new_gen);
+
+			continue;
+		}
+
 		old_gen = folio_lru_gen(folio);
 		if (old_gen < 0)
 			folio_set_referenced(folio);
 		else if (old_gen != new_gen)
-			__set_bit(i, bitmap);
+			folio_activate(folio);
 	}
 
 	arch_leave_lazy_mmu_mode();
-	rcu_read_unlock();
+	mem_cgroup_unlock_pages();
 
 	/* feedback from rmap walkers to page table walkers */
 	if (suitable_to_scan(i, young))
 		update_bloom_filter(lruvec, max_seq, pvmw->pmd);
-
-	if (!walk && bitmap_weight(bitmap, MIN_LRU_BATCH) < PAGEVEC_SIZE) {
-		for_each_set_bit(i, bitmap, MIN_LRU_BATCH) {
-			folio = pfn_folio(pte_pfn(pte[i]));
-			folio_activate(folio);
-		}
-		return;
-	}
-
-	/* folio_update_gen() requires stable folio_memcg() */
-	if (!mem_cgroup_trylock_pages(memcg))
-		return;
-
-	if (!walk) {
-		spin_lock_irq(&lruvec->lru_lock);
-		new_gen = lru_gen_from_seq(lruvec->lrugen.max_seq);
-	}
-
-	for_each_set_bit(i, bitmap, MIN_LRU_BATCH) {
-		folio = pfn_folio(pte_pfn(pte[i]));
-		if (folio_memcg_rcu(folio) != memcg)
-			continue;
-
-		old_gen = folio_update_gen(folio, new_gen);
-		if (old_gen < 0 || old_gen == new_gen)
-			continue;
-
-		if (walk)
-			update_batch_size(walk, folio, old_gen, new_gen);
-		else
-			lru_gen_update_size(lruvec, folio, old_gen, new_gen);
-	}
-
-	if (!walk)
-		spin_unlock_irq(&lruvec->lru_lock);
-
-	mem_cgroup_unlock_pages();
 }
 
 /******************************************************************************
-- 
2.39.0.314.g84b9a713c41-goog

Re: [PATCH mm-unstable v1 0/7] mm: multi-gen LRU: improve

[Yu Zhao]

On Tue, Jan 17, 2023 at 5:18 PM T.J. Alumbaugh <talumbau@google.com> wrote:
>
> This patch series improves a few MGLRU functions, collects related
> functions, and adds additional documentation.
>
> T.J. Alumbaugh (7):
>   mm: multi-gen LRU: section for working set protection
>   mm: multi-gen LRU: section for rmap/PT walk feedback
>   mm: multi-gen LRU: section for Bloom filters
>   mm: multi-gen LRU: section for memcg LRU
>   mm: multi-gen LRU: improve lru_gen_exit_memcg()
>   mm: multi-gen LRU: improve walk_pmd_range()
>   mm: multi-gen LRU: simplify lru_gen_look_around()
>
>  Documentation/mm/multigen_lru.rst |  78 ++++-
>  include/linux/mm_inline.h         |  17 -
>  include/linux/mmzone.h            |  13 +-
>  mm/memcontrol.c                   |   8 +-
>  mm/vmscan.c                       | 534 ++++++++++++++++--------------
>  5 files changed, 360 insertions(+), 290 deletions(-)

LGTM, thanks.

The design doc is still missing two sections (PID and mm_struct list),
and someone from our team will follow up on that.